This invention relates to a gas sensing element utilized for controlling the combustion of an internal combustion engine for an automotive vehicle or the like.
To control the combustion of an automotive engine, a gas sensor is installed in an exhaust system of an automotive engine. The gas sensor comprises a gas sensing element arranged in the following manner.
The gas sensing element comprises a solid electrolytic substrate having oxygen ion conductivity, a measured gas side electrode provided on a surface of this solid electrolytic substrate so as to be exposed to a measured gas, and a reference gas side electrode provided on another surface of the solid electrolytic substrate so as to be exposed to a reference gas.
Furthermore, the measured gas side electrode is covered by a porous protecting layer. The reference gas side electrode is provided in a reference gas chamber storing the reference gas. A heater, integrally formed with the solid electrolytic substrate, promptly increases the temperature of the gas sensing element to a predetermined activation level capable of detecting a gas concentration.
The role of the electrode protecting layer is to protect the electrode from heat of a measured gas and contamination by poisonous substances.
Furthermore, the electrode protecting layer has a role of promoting the diffusion of a measured gas to advance the reaction of HC and CO contained in the measured gas on a measured gas side electrode with O2 supplied from a reference gas side electrode.
However, providing the electrode protecting layer solely for the purpose of protecting the electrode from poisonous substances will increase the flow resistance of the measured gas flowing therethrough and accordingly worsen the response of a gas sensing element.
On the other hand, providing the electrode protecting layer solely for the purpose of improving the response will fail in preventing the poisonous substances from passing through the electrode protecting layer and accordingly deteriorate the output of a gas sensing element due to contamination of the electrode. Thus, the endurance of the gas sensing element will worsen.
Unexamined Japanese patent publications Nos. 60-228955 and 8-5603 disclose conventional manufacturing methods relating to an electrode protecting layers provided on a measured gas side electrode.
The former patent application discloses a method for manufacturing a gas sensing element having porous layers provided on the surfaces of a sensing portion and a heater portion of the gas sensing element, thereby preventing the gas sensing element from bowing during a sintering operation.
The latter patent application discloses a method for manufacturing a gas sensing element having a porous electrode protecting layer provided on a solid electrolytic substrate with a contraction rate of the electrode protecting layer being equalized with that of the solid electrolytic substance, thereby preventing the gas sensing element from bowing during a sintering operation. To this end, the material for the electrode protecting layer and a surface to volume ratio of the electrode protecting layer are regulated. However, it is difficult for these conventional manufacturing methods to surely satisfy both of the response and endurance requirements. Accordingly, it is earnestly required to provide an excellent gas sensor capable of satisfying such requirements.
Furthermore, unexamined Japanese patent publication No. 10-221287 discloses a gas sensing element comprising a structural member having gas permeation holes. However, even for this prior art, it is difficult to surely satisfy both of the response and endurance requirements.